Worn rings on ysm8 yanmar?

[Atcowboy]

I have a 1979 CD27 with a ysm8 with an unknown amount of hours. Currently running saw 30 per yanmar recommendation.

The engine seems to always smoke a little, more under load or higher rpms. Smells like burnt oil to me. When I pull the dip when running, it seems like there is a lot of pressure in the crank I get a good oil spray. Under full throttle there was some smoke around the cylinder head, I don't know if there is a breather there.

The engine runs okay. Any suggestions? Can rings on this engine be changed relatively easily, maybe in situ? Suggestions welcome.

- AT

[Wotname]

Originally Posted by Atcowboy

I have a 1979 CD27 with a ysm8 with an unknown amount of hours. Currently running saw 30 per yanmar recommendation.

The engine seems to always smoke a little, more under load or higher rpms. Smells like burnt oil to me. When I pull the dip when running, it seems like there is a lot of pressure in the crank I get a good oil spray. Under full throttle there was some smoke around the cylinder head, I don't know if there is a breather there.

The engine runs okay. Any suggestions? Can rings on this engine be changed relatively easily, maybe in situ? Suggestions welcome.

- AT

There is a breather, on the far side of the engine opposite the head. There will always be a pulsating pressure in the crankcase of a single cylinder engine unless the breather is huge, which it isn't. The pulsating pressure is created by the single piston moving in and out of the cylinder.

The YSM 8 is designed to be able to remove the piston in situ - assuming you have some space either side of the engine. You have to remove the head and the crankcase cover on the other side. This allows access to the big end bolts. Remove the big end cap and push the piston out through the head side of the cylinder. Reverse the process on refitting.

If it starts easily when cold (say within two seconds of cranking), then the rings aren't too bad.

Technically the sleeve can be removed in situ but that does take some serious effort.

[Atcowboy]

Originally Posted by Wotname

There is a breather, on the far side of the engine opposite the head. There will always be a pulsating pressure in the crankcase of a single cylinder engine unless the breather is huge, which it isn't. The pulsating pressure is created by the single piston moving in and out of the cylinder.

The YSM 8 is designed to be able to remove the piston in situ - assuming you have some space either side of the engine. You have to remove the head and the crankcase cover on the other side. This allows access to the big end bolts. Remove the big end cap and push the piston out through the head side of the cylinder. Reverse the process on refitting.

If it starts easily when cold (say within two seconds of cranking), then the rings aren't too bad.

Technically the sleeve can be removed in situ but that does take some serious effort.

She starts in just a few seconds I would say. I apply a little throttle preemptively, because she won't stay running if I don't. I think she's a little laggy, takes a while to spin up loaded or freewheel, but this is the only small diesel I've ever owned so I don't have much experience.

Reading through the manual a little more it does look like something that could be done in situ. I also looked a little closer and it looks like there is a breather hose that runs across the engine to the head, but it was attached. I remember it blowing off earlier this year and having to slide it back on. The head might have just been hot. It always looks a little oily, like maybe some fuel is weeping on it. Get it hot enough it can smoke...

I've noticed that there are at least aftermarket resleeving kits available online, that come with rings. I wonder if the cylinder is glazing over. I mostly run it to charge. I give it a little sauce every once it a while to try to treat her proper but she spends a lot of time just a little bit off idle.

I wonder if a little marvel mystery would help.

- AT

[Wotname]

You could pull the breather hose off the head end and redirect the hose into a tin can for awhile and see how much oil is caught in the can. I assume like many YSM8s, there is no air cleaner. If there is, bung off the breather hole at the air intake.

A single cylinder will always spin up slower than multi cylinders and the flywheel weight in the YSM8 is considerable compared to the hp out so that slows the spin time down as well.

If it using oil then clean out the exhaust / mixing elbow and give it a hard run, really hard for say 5 hours or more.

This may reduce the oil burn, not guaranteed though.

Do you have a tach, if so, what rpm can you achieve at WOT with a clean bottom and prop on a calm day?

[Atcowboy]

Originally Posted by Wotname

You could pull the breather hose off the head end and redirect the hose into a tin can for awhile and see how much oil is caught in the can. I assume like many YSM8s, there is no air cleaner. If there is, bung off the breather hole at the air intake.

A single cylinder will always spin up slower than multi cylinders and the flywheel weight in the YSM8 is considerable compared to the hp out so that slows the spin time down as well.

If it using oil then clean out the exhaust / mixing elbow and give it a hard run, really hard for say 5 hours or more.

This may reduce the oil burn, not guaranteed though.

Do you have a tach, if so, what rpm can you achieve at WOT with a clean bottom and prop on a calm day?

It has what I'm assuming is an "inertial filter" I'll call it. It's a largish can that I assume changes the flow direction to settle out larger particles. I can try seeing if I catch anything from the breather tomorrow, but how much is too much?

Also of interesting note, I check the oil level each day before starting and it doesn't budge. It's exactly on the oil line every time.

While I can't guarantee the mixing elbow is clean today, it is a new stainless unit installed 05/2021. The old one was black inside but not really carbon caked, I think it's demise was more due to rust...

Unfortunately, I do not have a tach, and Cailín's bum is not very clean right now either. I just go by speed and sound. Cruising speed seems about 3.5kts, which is when the engine smooths out and stops shaking the boat. WOT is about 4.25-4.5kts. It doesn't SOUND like it's over speeding at WOT, or lugging, seems pretty smooth, but my sense is this is running it pretty hard and hot. Again limited experience.

They last time I ran the engine similar to what you described was a few days ago, about 5 hours of motoring, but mostly at cruising speed. I did leave it WOT for a while, maybe 10 min or a little more. When I cut to idle it stalled and was very difficult to get started again.

- AT

[Compass790]

Have you got the service manual? I have the same engine, Wottie has given you good advice, Ill just add that worn valve guides cause a little blue smoke on ours but they got worn from lubrication failure. Good idea to check your oil pump clearances as per service manual. Where in the world are you Atcowboy?


Your speeds sound slow, we are overpropped in a 7.9metre boat & can get 5 knts out of it & 4.5knts cruising but I reckon we could get close to 6knts if we had the correct prop.
Mind you, hard to say what's happening with yours with no rev counter & i dont know yr boat design

[Wotname]

Starts OK when cold, doesn't burn oil; I'd say the rings and bore are just fine. Valves are probably OK as well although it won't hurt to check the valve lash (tappet clearance).

Are you operating in salt water or fresh?

Have you been replacing the engine anode regularly?

You probably can't tell if it is running hot but if you had an IR thermometer, you shoot the water jacket temperature and check it is below 60C (it should be).

Have you checked the thermostat and overheat switch? Both easy to do at home.

I don't know your boat design but at 27', I am thinking you don't have much reserve power meaning with any increase drag (dirty bottom / prop), you could be overfuelling the engine a little when operating at WOT.

Compass790 makes a good point about checking the oil pump, they are a problem on this engine. You could attach an oil pressure gauge to confirm it's OK.

[Compass790]

Have you ever checked the injector spray pattern?
Ours starts instantly & idles fine but I rebuilt it about 350hrs ago & it has over 400 psi compression now but before it was about 280. Sounds good yours doesnt use any oil, ours does use a little but again its leaking thru the guides.

What colour is the smoke, I'm guessing it's black.
You might have an air leakage problem too so check all yr fuel line connections & filters to help with idling difficulty.
If you are operating in salt water an acid flush would be a good idea.
Before our ysm8 was rebuilt we would have to use full throttle & a little engine oil in air intake to get it to start but just new rings made it start with no engine oil.
I did however replace piston & liner when I pulled the engine out to replace flogged out conrod.
Good you got an ss exhaust elbow, much better idea than cast iron.

[Atcowboy]

Originally Posted by Compass790

Have you got the service manual? ... Good idea to check your oil pump clearances as per service manual. Where in the world are you Atcowboy?
Your speeds sound slow, we are overpropped in a 7.9metre boat & can get 5 knts out of it & 4.5knts cruising but I reckon we could get close to 6knts if we had the correct prop.
Mind you, hard to say what's happening with yours with no rev counter & i dont know yr boat design

I do have the service manual, and have been leafing through it.

I keep the boat in Mount Hope Bay MA/RI.

Those speeds sound fantastic to me, especially for a 25' boat, I generally consider a good clip for my 27' (8.2 meter) boat under sail to be around 6kts avg. The Cape Dory 27 has a "fin keel", so a full keel with a cut away forefoot and the rudder hung on the keel. Factory listed displacement is 7,500lbs, and I have her loaded up for bear as I pretend to be a full time cruiser. She rides pretty low on her lines, hence the dirty bum (I haven't adjusted the antifouling line up yet). I have no reason to suspect the prop was ever changed from the factory (but that doesn't necessarily mean it's the "correct" prop).

CDSOA, Inc. -- CD27

Originally Posted by Compass790

Have you ever checked the injector spray pattern?
...
What colour is the smoke, I'm guessing it's black.
You might have an air leakage problem too so check all yr fuel line connections & filters to help with idling difficulty.
If you are operating in salt water an acid flush would be a good idea.
Before our ysm8 was rebuilt we would have to use full throttle & a little engine oil in air intake to get it to start but just new rings made it start with no engine oil.
I did however replace piston & liner when I pulled the engine out to replace flogged out conrod.
Good you got an ss exhaust elbow, much better idea than cast iron.

I have never pulled the injector nor had it checked in any way.
The smoke is not black, I haven't ever seen it black as far as I can recall. It is pretty much just white, I don't really even see that bluish haze you would expect from oil burn but I suspect oil burn because that's what it smelt like, at least yesterday.

Both you and Wot have suggested looking at the cooling system, would that affect the engine in a way that it would produce smoke?

If idle problems when throttling down indicate a possible air leak, then I guess I should take a look. It only seems to have a problem when throttling down. Sometimes I wonder if I have my idle set too low, I had to replace the remote unit and idle is again only set by ear. When throttling back if I leave it just a touch above idle it seems to do "alright".

Originally Posted by Wotname

Starts OK when cold, doesn't burn oil; I'd say the rings and bore are just fine. Valves are probably OK as well although it won't hurt to check the valve lash (tappet clearance).
Are you operating in salt water or fresh?
Have you been replacing the engine anode regularly?
You probably can't tell if it is running hot but if you had an IR thermometer, you shoot the water jacket temperature and check it is below 60C (it should be).
Have you checked the thermostat and overheat switch? Both easy to do at home.
I don't know your boat design but at 27', I am thinking you don't have much reserve power meaning with any increase drag (dirty bottom / prop), you could be overfuelling the engine a little when operating at WOT.
Compass790 makes a good point about checking the oil pump, they are a problem on this engine. You could attach an oil pressure gauge to confirm it's OK.

I have never checked the valve clearances, although I should have what is needed to do so on the boat.

I am in salt water, and as far as I know the boat has only ever lived in salt water.

I replaced the anode last season before launch. I checked the anode before launch this year. It had eroded about 20-30 percent, so I left it in, thinking 50% would be the end of this season.

I've been procrastinating on buying a non-contact thermometer but this was enough to make me pull the trigger. Once I have the thermometer in hand, what part of the engine best represents the water jacket temperature?
I have never pulled the thermostat, but the boat came with a spare. Never having boiled it to check when it opens, I THINK it works.
I just checked the water circuit, the light turns on when shorted to ground. I imagine the test for the switch would be similar to a thermostat; boil it? I have not done that.

Both you and Compass have suggested looking at the cooling system, would that affect the engine in a way that it would produce smoke?

What does over-fueling look like, white smoke? I guess that makes sense as I think poor injector pattern can produce white smoke (unburnt fuel getting fogged I think it is). It is definitely less smoky when not WOT. The smoke is hardly noticeable at idle.

I purchased an engine gauge kit a while back so I could see the oil pressure, but I've been procrastinating getting it installed. I'll try to do that today.

- AT

[Halifax Sailor]

Hello Atcowboy,

Take these guys advice!! I have a ysb8, circa 1978 and had issues with hard start. They provided so much advice that I followed and now mine starts immediately with no lag in throttle. Will push my boat easy at 4.5 knots and I can get 6 knots with 3000 rpm. I had changed exhaust elbow, all fuel lines, and reset valve clearances to get it to start correctly. Thx to these guys I have a great running engine again!!

[Atcowboy]

Some new diagnostic news;

Engine started up easy again today.
Assuming an accurate oil gauge, oil pressure immediate jumped to 25psi, and climbed to ~30psi at idle in 20 seconds or so. After warming up, climbs to 37psi at WOT. Seems like some oil pump work might be warranted. I think the manual is saying 35psi should be idle pressure?

No smoke to speak of at startup, or WOT without load. I then did a WOT pull on my mooring in reverse and the smoke and smell came back. Throttled down and did a WOT run under no load and the smoke cleared pretty quick. Each unloaded WOT produces a small puff but when at WOT is relatively clear. I'm not sure what my prop looks like (diving in dark water makes me feel claustrophobic).

Sounds like I have some maintenance points I can/should check, but nothing dire? I was thinking I was looking at a major rebuild or a repower... Phew...

Rebuild oil pump (soon)
check valve clearances for fun
maybe have a shop check the opening pressure on the injector at some point
Maybe check compression for fun?

Off topic to this but are there any higher output alternator options for this engine that work well? I believe it still has the original 30 amp alternator regulated for lead acid when I have AGMs.

- AT

[Compass790]

Ok Atcowboy,
after your latest diagnostic news I'd say you probably dont have too much of a problem.


The cooling system prompts are more on the maintenance side, you could get more white smoke when cold if your thermostat sticks open. From my point of view it was just warning you that the galleries can get clogged with salts buildup over the years & then you get isolated hot spots.
Best to follow the manuals checks although they are OTT on some things like replacing fuel lines every 4 years. Maybe if the engine was powering a helicopter not a yacht ( sailboat in American "English" )
I would definitely at least check the injector if it never has been. In fact I'd just throw a new nozzle in it, they are cheap on Ebay & easy to install. PM me if you want the link. I wouldnt even worry about a pop-test if you re-install the old shims it should be within coo-ee


Oil pressure doesnt sound too bad, nothing to panic over. Do you spin the oil filter handle before start as described in operating instructions? I doubt your oil pump needs rebuilding but just check tolerances with feeler gauges as described. The rotor can wear down into the case. Report back if rotor to face of case tolerance is too big & I'll advise on cure.


The prop should be a 12x8 IIRC but check manual.
I looked at your boat on sailboat data & suspect we would have a longer waterline & also a considerably less drag hull form.


The ys series Yammers always puff black smoke when you shove the throttle fwd & take a little time to clear.


We have a Bosch 55amp alternator on a custom mount that came with boat ( tho not working ) as you have to have 2 alternators to get Cat 1 clearance to go offshore.
The Hitachi 35 amp original is pretty useless to us as we have a pair of 225 amp 6v deep cycles in series to charge & the Hitachi has a thermal cutout switch that keeps turning it off to stop it from burning out. Great at saving the alternator but crap for getting the batteries full. I stripped the 55 amp Bosch & greased the bearings then fitted a 14.8v regulator with a current control on the field wire so I can regulate the alternator load. We cant have the alternator fully powered up when using the engine for propulsion as it gets into an overfuelling/overloaded state & blows black smoke.
This is not helped by being overpropped.
The 7hp ( continuous) engine is very sensitive to a dirty bum or a head wind/sea not to mention alternator load as you would have noticed.


Definitely check your valve clearances too if they havent ever been done

[Compass790]

Atcowboy Wotname & I are giving you tips from our experiences in dealing with worn out examples of these engines to help you avoid failure points.
You may like to read my post from the start of my ysm8 education
https://www.cruisersforum.com/forums...-a-148914.html
CF call me a marine service provider as i sell the occaisional aftermarket part for them so have learnt quite a bit about failure points from the owners that contact me.
Quite a few of the engines get dumped here in NZ as the parts prices make it uneconomic to rebuild if you buy genuine parts from the Yanmar dealer& pay a mechanic to do the rebuild.
Not so bad in the USA as parts are 1/2 NZ dealer price.

I think it's a shame to trash them for that reason & many of the owners cant afford a new engine as it would cost more than their boat.
The most common causes of wear problems are ( apart from dirty wet fuel or oil )
exhaust elbow failure that dumps water on the top of the exhaust v/v & then gets into the bore. Next up is oil pump wear.
I believe the engines would be extremely long lasting if well maintained & fitted with an ss elbow with the water injection on the downhill side so it cannot squirt salt water on top of the exhaust v/v.
Your engine sounds pretty good for its age considering it hasnt been rebuilt.
Should you do a compression check please report back what it is as I'm curious.

[Wotname]

Atcowboy, I think Compass790 has covered all the salient points, nothing left for me to say but hey, I won't let that stop me.

I agree the oil pressure is OK, not great but OK for a 40+ year old engine. While improving it is doable, it ain't super easy. Compass790 knows the tricks!

Best spot to shoot the engine temp is around where the overheat switch or anode is. It should be below 60C. The reason I mention to check the overheat switch is simple - it could be 40 years old and should it fail (and they do) and should the engine overheat, you won't know until too late. IMO, they should be checked every few years or at least every decade but very few do.

One inherent problem with a horizontal cylinder is that any condensation that occurs when sitting collects lies along the bottom surface of the cylinder and can cause corrosion. A good trick to minimise this possibility is to (after shutting it down) rotate the engine over by hand until it reaches the compression stroke. Do this whenever leaving it sitting for more than a few days. This ensures the valves are fully closed and the space in the cylinder is minimised. Obviously this only works for a single cylinder engine!

[Atcowboy]

I remember to turn the oil filter "often" but I'm not every time. Maybe I should change leaving my keys in the engine block (to ensure I open the seacock) to hanging on the filter wing nut. I'm reading through some (older) posts from you guys about remote filters right now. What are your current thoughts on them?

So I'm not back at the boat until at least tomorrow, and I haven't measured the pump clearances. If the pump has eaten into the case, what is the remedy? I'd like to have an idea what I'm getting into with that. When it got really warm, it was 25psi at idle. Sometimes at idle you can hear the alarm "squeek" just barely once per revolution. I've heard of epoxy build ups to repair water pumps on cars after a rotor ground into a crank case.

I'm also running straight SAE30. I don't bother trying to track hours on it but it must be close enough to a change interval for at least new SAE30. But maybe a multi weight could help get the pressure up a little without reworking?

- AT